structural FOUNDATIONS Pile Load Testing for Bored Piles in Soil A Brief Summary By Hee Yang Ng, MIStructE, C.Eng, P.E. ored piles, also referred to as auger-cast piles, are large-diameter at intervals throughout the length of the pile so that the force for cast-in-place concrete piles used commonly to support buildings each segment can be calculated and the pile’s load transfer can be Bwhen column loads are high. The design of bored piles requires the obtained. The same pile installation method should be used to install designer or the project’s geotechnical engineer to the working piles for the project. To adequately estimate the load-carrying capacity of the pile based verify the shaft friction and end-bearing capacity, on the ground conditions at the project site. One preliminary piles are typically loaded to 3 times way to predict the pile capacity is by carrying out the working load. calculations using soil parameters obtained from Working pile tests are conducted during piling site investigation data. While many pile design work to verify the quality and workmanship of methods are highly empirical, pile capacity can also the installed piles. These tests verify that installed be affected by other factors such as the method of piles perform as-designed to meet strength (abil- installation, quality of workmanship, and construc- ity to carry load) and serviceability criteria (with tion materials used. Therefore, pile load testing is acceptable settlement). Working piles are loaded an essential aspect of pile design that should not progressively, with the load and pile head settle- be overlooked. ment monitored and recorded. The acceptance This article summarizes pile load testing (static criteria for working piles are settlement limits at type using a kentledge) for bored piles in soil and Figure 1. Generalized load-settlement certain loads. For example, in CP4 (SS CP4 Code highlights some key aspects designers should look curve of a pile. of practice for foundations, Singapore Standard), for when reviewing and interpreting pile load test the settlement cannot exceed 0.59 inches (15mm) results. For more information on pile design, the at 1.5 times the working load or 0.98 inches reader is directed to Pile Structural Capacity – A (25mm) at 2 times the working load. The higher Comparison of Three Design Codes published in the allowable value of settlement at 2 times of work- March 2020 issue of STRUCTURE. ing load is to recognize that, at higher loads, the rate of increase in settlement is likely to be greater. Recommendations for the number of tests are Types of Load Tests shown in Table 1. There are essentially two main types of pile load tests: the preliminary pile test and the working pile test. A preliminary pile test is typically carried Load-Settlement Curve out before the piling works commence while the A simplified load-settlement curve is shown in Figure 1. working pile test is carried out during piling or Generally, the curve is non-linear, and the stiff- when substantial piling works are completed. When ness of the response tends to degrade or soften as using limit state design (e.g., Eurocode 7-EC7), the the load gets higher. This is expected because, as preliminary pile test and working pile test allow the load increases, the pile approaches “failure” in designers to adopt less conservative partial factors Figure 2. The difference in mobilization geotechnical terms. Some codes define the “failure” of shaft friction and end-bearing. when reducing unfactored shaft friction and end- of a pile as reaching a settlement of 10% of pile bearing, resulting in a more economical design. Similarly, when using diameter. For large diameter piles, this criterion becomes somewhat LRFD design or ASSHTO specifications, more load testing may allow lenient if the serviceability limit state is considered. For example, a a higher resistance factor (φ) to be used due to the increased reliability. 39.37-inch-diameter (1000mm) bored pile has an allowable settle- The preliminary pile test verifies the design parameters prior to the ment of 3.94 inches (100mm). The settlement acceptance criterion start of piling work. Therefore, the test pile is instrumented so that has to be stringent because pile settlement can result in a differential assumed values of shaft friction and end-bearing could be verified. A settlement between supports, which is a grave concern due to the borehole close to the pile location is necessary to ascertain the type amplification of tilt for a tall superstructure. In addition, build- and layering of the soil for verification. Strain gauges are installed ing settlements may disrupt utilities and services (e.g., pipes and cables). Therefore, a much smaller value of the allowable settlement Table 1. Number of pile load tests. is usually required. Piles rarely “fail” with regard to vertical structural capacity. In a case Preliminary pile test 1 pile or 0.5% of the total number of working piles, whichever is greater. study presented later, a quick calculation shows that the axial stress in the pile at its rated working load is on the order of about 8 to 10 Working pile test 2 piles or 1% of the total number of work- MPa (or 1.2 to 1.5 ksi). ing piles or 1 for every 50 meters length of A critical aspect of pile design is shown in Figure 2, where the proposed building, whichever is greater. load on a pile installed in soil is resisted by a combination of shaft 52 STRUCTURE magazine friction and end-bearing. As the pile is loaded, shaft friction increases pile with its load-settlement path running along the linear-elastic gradually and can become fully developed by a small movement portion of the curve will likely return to zero load with minimal of approximately 0.5% of pile diameter, say residual settlement. 0.2 to 0.4 inches (5mm to 10mm). However, Sometimes it can be helpful to compare the gradient a large movement is required for end-bearing of the load-settlement curve against the elastic short- to be fully developed, typically in the range of ening gradient (PL/AE line). For example, the elastic 5% of pile diameter. This means that to achieve shortening of a bored pile without any confinement its full end-bearing value, the pile must settle is given by PL/AE, where P is the applied load, L is more, which may not be desirable. For this the length of pile, A is the cross-sectional area, and E reason, designers tend to select more conserva- is Young’s Modulus. tive values for end-bearing, and codes impose The load-settlement gradient of a friction pile a higher factor of safety. should generally be similar or even very slightly Figure 3 shows some examples of load-settle- steeper compared to the elastic shortening gradient. ment curves of piles with anomalies compared This is because of the presence of soil confinement to a pile expected to perform normally. The and shaft resistance. However, suppose the gradient red curve is abnormal because of kinks in the of the load-settlement of a pile is gentler than the curve, possibly resulting from pile defects or Figure 3. Examples of “abnormal” elastic shortening gradient. In that case, it may be load-settlement curve of a pile. unexpected ground conditions that cause the a case of less friction being developed (e.g., softer pile to settle abruptly when loaded. This inability to sustain load at soils) along the shaft or a case of stiffer soils located only at the localized portions of the curve is not acceptable. The purple curve lower end of the pile. shows normalcy up to a point and degrades abruptly, finding a plateau with a lower load and increasing settlement. This might occur if the pile base is poor or suddenly loses support coupled Calculation of Pile Settlement with a loss of a portion of shaft friction or deterioration of shaft There are various methods to estimate anticipated pile settlement in friction. One example is a case of cavities, archetypal in limestone design. One of the easiest ways to calculate pile settlement by hand is areas. Lastly, the blue curve is abnormal due to its inability to reach by Vesic (1977), as shown in Figure 5 (page 54). In this method, the the expected design load and experiences excessive settlement too pile head settlement is the sum of three components, namely axial early. This might occur for a pile that has fully maximized the shaft compression (ws), the settlement at pile toe due to shaft load (wps), friction and cannot develop the end-bearing, resulting in a “plung- and settlement at pile toe due to end-bearing load (wpp). ing” type of failure. Axial compression in soil can be simplified by assuming it as 75% of unconfined elastic shortening (PL/AE). The formula given by Vesic uses the sum of end-bearing load and a fraction (0.5 to 0.67) Case Study of shaft friction load in place of axial load in the unconfined elastic A 31.5-inch-diameter (800mm) bored pile was installed to a depth shortening formula to calculate the axial shortening of the pile in of 85.3 feet (26m) below ground in alluvial soil. These soils were soil. Settlements due to shaft load and end-bearing load are given by formed by transported material deposited and cemented over time. the ratio of shaft design working load (Qs) and end-bearing design The working load for such a pile is 450 tons (4000 kN). Please note working load (Qb) against the ultimate bearing capacity at the pile that pile capacities provided by geotechnical engineers are always toe (Qo), normalized by the relevant dimension (pile length L and stated as allowable loads (ASD) or working loads, typically with a pile diameter d, respectively) and multiplied by Cp and Cs, empirical safety factor of two against a geotechnical failure.